The present invention generally relates to a cellulose nanocrystal, and in particular, to a method of forming a substantially uniaxially-oriented high content cellulose nanocrystal film with improved characteristics and properties.
According to the United States Department of Energy and Agriculture, approximately one billion tons of biomass, i.e., any biologically derived material, can be produced annually in the United States. Cellulose, the world's most abundant natural, renewable, biodegradable polymer, can be used to strengthen plastics, provide a lightweight component and advantageously is biodegradable. Cellulose nanocrystals (CNC), which are the primary structural unit for plant life, can be extracted from plant biomass (e.g., trees, grasses, cotton, sisal, bamboo and ramie). Cellulose nanocrystals can also be found as structural components in tunicates (sea creature similar to sea cucumbers), and are produced naturally by the acetobacter xylinum bacteria. In addition to being used in plastics, the cellulose nanocrystals can be used in ceramics and in biomedical applications such as artificial joints and disposable medical equipment. These nanocrystals provide several advantages over glass including being lighter weight, easier on processing machinery, less expensive to work with, and breaks down quickly in a landfill, for example.
Due to the inexpensive, renewable nature of cellulose nanocrystals, as well as their exceptional mechanical properties, their use as a reinforcement phase in polymer based composites has been a popular topic of recent research. However, a majority of the current research has focused on low fractions of cellulose nanocrystals, typically less than 20 wt. %, to improve the properties of various polymers. The high modulus of the cellulose nanocrystals suggests that CNC phase-dominant composites (>50 wt. %) could serve potential high-strength applications, as well as an increased “green” aspect.
CNC morphology (length, aspect ratio, length polydispersity) and surface charge vary greatly based on synthesis conditions. Typically, acid hydrolysis is used to break down cellulose microfibrils by digesting the amorphous regions that connect cellulose nanocrystals. The overall process typically requires heating, agitation, rinsing, filtration, dialysis, and ultrasonication, with the parameters of each step having a direct impact on CNC morphology and/or surface chemistry. Therefore, researchers have attempted to determine the effects of each processing step on cellulose nanocrystal properties. The final result of CNC processing is almost always the same, in that a suspension of colloidal liquid crystalline cellulose nanocrystals is produced, forming either a nematic or chiral nematic mesophase (dependant on CNC length, aspect ratio, length polydispersity, surface charge, CNC concentration, and electrolyte concentration).
Another area of recent CNC research has focused on orienting cellulose nanocrystals in suspension, over large domain sizes as opposed to the small domains of orientation that develop due to mesophase formation. Several methods of inducing orientation have been investigated, generally involving either a large magnetic field or electric field. The magnetic and electric field options can require significant energy input that detracts from the “green” aspect of cellulose. In addition, magnetic fields do not orient individual cellulose nanocrystals uniaxially, but instead orient the chiral nematic domain axes of the CNC mesophase (with cellulose nanocrystals oriented perpendicular to magnetic field lines).
Most conventional processes for forming a CNC film use a high quality tunicate or bacterial nanocrystal, rather than plant-biomass-based, because the former are pure, have greater length, and are easier to obtain. In addition, cellulose nanocrystals derived from wood generally display worse behavior, e.g., are more difficult to orient. Further, past research using cellulose nanocrystals derived from tunicate or bacteria has produced advantageous mechanical properties including strength and modulus.
Once formed, retaining CNC uniaxial orientation induced during a shear casting process in the final dried CNC film is challenging. A time- and rheology-dependant relaxation occurs once shear is removed and the uniaxial CNC orientation dissipates. To counter this relaxation, conventional CNC films have been dried under a constant rotational shearing process and achieved better uniaxial orientation. However, these dried films are cylindrical, which severely limits potential applications and complicates mechanical property characterization. Other conventional processes have used drawing methods to pull a semi-dry film of bacterial cellulose fiber into alignment and held the strain during a drying process. This conventional process has produced a uniaxial orientation in a flat film, but has been most effective for high aspect ratio (<100) fibers that produce a CNC gel-like network structure that can be mechanically deformed in the wet state.
Thus, there is a need for an industrially-relevant process of forming a flat, highly oriented film of high content readily available cellulose nanocrystals extracted from plant biomass. In addition, there is a need for a shear-based orientation process for forming the plant-biomass-based cellulose nanocrystalline film. Once formed, the mechanical properties of the highly oriented CNC film can then be used for potential applications in high-strength composites manufacturing.